1. Field of the Invention
The present invention relates to a power supply switching circuit, and more particularly to a power supply switching circuit with a protection function, which can selectively receive power from a built-in secondary battery cell and power from an external power supply.
2. Description of the Related Art
There is known, electronic equipment which can operate based on power supplied from an external power supply and power supplied from a secondary battery built into the electronic equipment. In such electronic equipment, it is possible than when power is supplied from the external power supply, an; over-voltage which exceeds an adequate charging voltage for the secondary battery is applied to the secondary battery. In this case, the secondary battery is not charged properly. Therefore, there is the possibility that the electronic equipment is set in a dangerous state due to generation of heat and so on.
In order to avoid such a dangerous state, the electronic equipment may include a switching circuit for switching a power supply source between the secondary battery and the external power supply. Such a switching circuit may be composed of a relay, a transistor switch, a mechanical switch or the like. Alternatively, the switching circuit might be realized by ON/OFF switches respectively provided in parallel for power supply lines for the secondary battery and the external power supply, and to turn on either one of the switches.
On the other hand, in the charging operation of a secondary battery cell, an over-voltage protection circuit or an over-discharge protection circuit may be provided to detect the above-mentioned extraordinary charging state to the secondary battery cell. The protection circuit would electrically disconnect the secondary battery cell in the event of an extraordinary charging state. It is desirable that these protection circuits are composed of a detecting circuit for detecting that the over-voltage state or the over-discharge state is present, and a switching circuit which is controlled based on the detection result by the detecting circuit to electrically disconnect the secondary battery cell.
However, if the power supply switching circuit and the secondary battery cell protection circuit are provided independently, there are multi-stage switch circuits in series in the power supply line of the electronic equipment. This becomes disadvantageous in the small size and weight of the electronic equipment. This problem is especially conspicuous in a portable communication terminal comprising the electronic equipment.
Also, the resistances of respective switch circuits are added on the power supply line. As a result, the series resistance of the power supply line increases, so that the line voltage descreases because of operation current to the equipment, and power supply noise increases, resulting in degradation of the performance of the electronic equipment.
The power supply switching method when AC power is supplied from the external power supply is disclosed in, for example, Japanese Laid Open Utility Model Disclosure (JP-U-Show a 58-645). In the method, the connection of the external power supply is detected and then a power supply line is switched. In this example, however, the charging operation protection of the secondary battery cell is not considered. When the charging operation protection is considered, because the double switch structure becomes necessary, the above-mentioned problems will be present.